Today, I noticed that Skype video was not working, and I suspect it's connected to changing phonon backend from vlc to gstreamer.
So I tried to add more capabilities to gstreamer by installing the ffmpeg links:

You are using a third party repo which also provides libavcodec53. You can see this by the package number starting with "5:".
We and Debian cannot do anything, please ask the person who is providing the repository you are using.

Double checked things. You are correct, that libavcodec53 is newer than the repository version.
However, I have _no_ relevant 3rd party repos in my sources.list (as said, I don't consider opera relevant for multimedia stuff):
http://pastebin.com/RCR1CdL3

I suspect there are newer packages left over from times when I used 3rd party repos (even when I can't remember having done that...).

Now, I want to get in sync with Debian sid again, so I face the challenge of downgrading a certain number of packages (I found at least libavcodec51/53, libavutils51, but there may be others).

Is there a way to put all packages back to a certain mirror's versions (I think the question is similar to going back to stable from unstable or such, and this might not be officically supported :-/ )?
At least, I think I have no packages that are _not_ available on my mirror, so just changing all versions to the officical "sid" ones would leave no unsolved dependencies...

* The ffmpeg program has been deprecated and replaced with avconv.
The ffmpeg program is only provided for compatibility purposes and
will be removed in a future release. For a list of the
incompatible changes between the two programs, see
/usr/share/doc/ffmpeg/changelog.gz.